Isoprene purification process



Jan. 18, 1966 A. B. HILL ETAL ISOPRENE PURIFICATION PROCESS Filed May 18, 1962 4 WATER l1 c FEED 5 "T" ,1 H 9 l5 ARTHUR B. HILL DAVID C. PARNELL HIGH PURITY l3 ISOPRENE WATER WATER NVENTORS PATENT ATTORNEY United States Patent i 3,230,157 ISOPRENE PURIFICATION PROCESS Arthur B. Hill, Westfield, N.J., and David Crosby Parnell, Baton Rouge, La., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed May 18, 1962, Ser. No. 195,710 12 Claims. (Cl. 203-53) This invention relates to an improved method for recovering isoprene of a high degree of purity from highly complex mixtures produced by steam cracking or other high temperature hydrocarbon conversion processes.

The problem of separating isoprene from C cuts, particularly those produced by the steam cracking of hydrocarbon, is rendered particularly diflicult because of the close relationship in properties of the several compounds and the pronounced tendencies of the more unsaturated hydrocarbons therein to polymerize. The following table lists the principal C hydrocarbons that are found in admixture with isoprene in a closeout C stream from a refinery cracked stock. The table also lists the boiling points at 760 mm. of these hydrocarbons and the relative volatilities (a) (relative to isoprene as one) which are a direct measure of the ease of separation in a distillation system.

Pentadiene 1-4 3 methyl Butene-1 Cis-peutene-2 Z-methyl Buten Cyclopentadiene- Propylacety1ene Allylacetylene 'lranspiperyleue, Cls-piperylene Cyelopenteue.

Cyclopentane Based on normal vapor pressures.

It may be seen that the boiling points and the relative volatilities of the various C compounds do not follow the degree of unsaturation. It is obvious, therefore, that fractional distillation alone as a means for separating isoprene from such complex mixtures is entirely impractical. It has been proposed to effect this separation by various combinations of straight distillation, extractive distillation, and/or azeotropic distillation. Typical combination processes of this nature may be found in US. Patent 2,361,493 of Patterson, US. Patent No. 2,426,705 of Patterson et al., and US. Patent 2,459,403. Other techniques proposed have included dimerization of the cyclopentadiene to facilitate the separation of isoprene as shown, for example, in Maisel US Patent 2,704,778. These procedures have not been entirely satisfactory either because they have been deficient in the rejection of acetylenes or have involved the rather ineificient dimerization step.

It is the object of this invention to separate isoprene of a high degree of purity from highly complex mixtures of highly unsaturated hydrocarbons of close boiling range.

It is a further object of this invention to provide a process for the recovery of isoprene of high purity by a combination of straight and extractive distillation steps which not only efiects adequate rejection of acetylenes but which also eflects adequate segregation of cyclopentadiene without resorting to dimerization.

3,230,157 Patented Jan. 18, 1966 These and other obejcts will appear more clearly from the detailed specification and claims which follow.

It has now been found that isoprene of high purity can be separated from a C fraction containing acetylenes and olefins as well as cyclopentadiene by first subjecting the fraction to straight distillation. The presence of C olefins in the straight distillation step forces isopropylacetylene, dimethylacetylene, pentadiene 1-4 and some of the valylene overhead. The C olefins enhance the volatility of those acetylenes relative to the volatility of isoprene. The bottoms from this straight distillation is then subjected to extractive distillation using a volatility modifying agent such as acetone or acetonitrile in two stages. In the first extractive distillation stage the C olefins are taken overhead and the extract is charged to the second extractive distillation stage. The high purity product isoprene is taken overhead from the second extractive distillation stage while the remaining valylene, cyclopentadiene, piperylene propylacetylene and allylacetylene are rejected in the extract phase. The hydrocarbons contained in the extract phase are then separated from the volatility modifying agent and the latter then conditioned for recycling to the first extractive distillation phase.

In order to illustrate the invention more clearly refer ence is made to the accompanying drawing which shows a diagrammatic flow plan of the process of this invention.

In the drawing a feed stock containing mainly C olefins, diolefins, and acetylenes (including dimethyl acetylene) obtained, for example, by fractionating a steam cracked hydrocarbon product is supplied via line 1 to a primary fractionator 2. This primary fractionator separates overhead through line 3, isopropyl acetylene, dimethyl acetylene, valylene, portions of the pentene-l, and Z-methyl butene-l and most of the pentadiene 14 as well as any C and lighter hydrocarbons from the feed stream. The overhead stream is condensed, collected in reflux drum 4 and part recycled to column 2 as reflux and part withdrawn via line 5. Column 2 should contain about 70 plates and should be operated at a reflux ratio of about 13:1, at a pressure of about 36 p.s.i.g. and with temperatures at the top of l38-140 F. and at the bottom of l72175 F.

The bottom stream from the primary or straight distillation column 2 is withdrawn through line 6 and charged to the first extractive distillation stage or column 7 for the separation of the remaining C olefins and remaining pentadiene 1-4 from the other diolefins and remaining acetylenes. This separation is effected by supplying the hydrocarbon stream to about the middle part of an extractive distillation column and supplying a suitable volatility modifying agent such as acetone or acetonitrile to the column above the hydrocarbon inlet as through line 8. Makeup and/or recovered volatility modifying agent is supplied through line 9. A stream of volatility modifying agent is withdrawn from the bottom of column '7 and part of it is passed through a Ieboiler and back into the lower part of column 7 while the major proportion of said stream is recycled to the top of column 7. The extractive distillation column 7 should contain about plates and should be operated at a reflux ratio of about 5:1. The solvent or volatility modifying agent should be circulated at a ratio of about 1 to 1 on total hydrocarbon in the column. Column 7 should be operated at about 30 p.s.i.g. with a temperature of about F. at the top and about 255 F. at the bottom.

The remaining C olefins and remaining pentadiene 1-4 are taken overhead from column 7 with a small amount of volatility modifying agent through line 10, condensed and charged to reflux drum 11. A part of the condensate is recycled to column 7 as reflux and the rem-aining portion is supplied through line 12 to the water Wash tower 13. The water wash tower 13 removes the volatility modifying agent, i.e., acetone or acetonitrile from the C olefins and pentadiene 1-4 and the latter are removed through line 14 to further processing elsewhere. Water and volatility modifying agent are withdrawn from the bottom of the water washer 13 and passed to a suitable recovery or concentrating system preparatory to recycling the agent to line 9 and thence to column 7.

The extracted isoprene product along with the remaining diolefins (piperylcne and cyclopent-adiene) and acetylenes is withdrawn from column 7 via line 15 and charged to the second extractive distillation stage or column 16. Additional volatility modifying agent is supplied to the top of column 16 via line 17 and the extractive distillation is conducted in said column to effect the separation of the isoprene from the remainder of the valylene, cyclopentadiene, p-ropylacetylene, allylacetylene and cisand trans-piperylene (pentadiene 13). The column 16 should contain about 70 plates and should be operated at a reflux ratio or" about 6: 1. The solvent or volatility modifying agent should be circulated at a rate of about 2 to 1 based on total hydrocarbon in the column. Column 16 should be operated at about p.s.i.g. with a temperature of about 120 F. at the top and about 215 F. at the bottom.

The isoprene and small amount of volatility modifying agent are removed overhead from column 16 via line 18, condensed and the condensate discharged into reflux drum 19 from which a portion of the condensate is recycled to the CO'IUIIHI]. 16 as reflux and the remaining portion is discharged via line 20' into water wash tower 21. The tower 21 removes the volatility modifying agent, i.e., acetone or acetonitrile from the isoprene and the high purity isoprene .product is then removed from the tower via line 22 for use as is or for further purification to remove any remaining piperylenes by simple fractionation. Water and volatility modifying agent are withdrawn from the bottom of tower 21 and passed to a suitable recovery or concentrating system preparatory to recycling the agent to column 16 via makeup solvent inlet line 23 or to tower 7 via inlet line 9.

The extract is Withdrawn from the lower portion of column 16 via line 24 and, if desired, passed through a stripper 25 for effecting some separation of hydrocanbons which are taken overhead from solvent or volatility modifying agent which is withdrawn from the bottom of the stripper and recycled to the column 16. The material taken overhead from stripper 25 is condensed and a portion of the condensate is recycled to stripper 2 as reflux. The remaining portion of the condensate is charged via line 26 to water wash tower 27 which serves to make final separation of the solvent or volatility modifying agents from the hydrocarbons. The washed hydrocarbons consisting principally of valylene, cyclopentadiene, propylacetylene, allylaceylene and cisand trans piperylene are removed via line 28 while water and volatility modifying agent are removed from the bottom of tower 27 via line 29 and passed to a suitable recovery and concentrating system preparatory to recycling the solvent to column 16 via line 23 or to column 7 via line 9.

The isoprene product removed via line 22 is of very (high purity, at least about 97 wt. percent of the total stream making it eminently suitable for the production of high quality polymeric products. A final straight distillation step can be used to increase the isoprene purity to 99.5 wt. percent.

The following example is illustrative of the present invention.

Example A representative feed charged to the system of the present invention is a stream cracked hydrocarbon fraction having the following composition (mol percent).

This feed is charged to a fractionator column 2 having 70 plates and operated at 36 p.s.i.g. with a temperature of 138 F. at the top and 172 F. at the bottom. The relative material flow rates are 7,000 #/hr. of feed, 24,700 #/hr. reflux, 1,900;#/hr. overhead product withdrawn and 5,100 #/hr. of bottoms product. The overhead distillate product removed through line 5 and the bottoms product removed through line 6 have the compositions (mol percent) shown in the following table.

TAB LE II Stream 5 2 Methyl Butene-l Pontpne 1 Pentadiene 1-4 Trans Pentene-2 Cis Pentane-2- C4 3 Methyl Butene-l Ois Piperylene Oyclopentadiene The bottoms stream from column 2 is charged to exractive distillation column 7 having plates and operated at 30 p.s.i.g. with a temperature of F. at the top and 255 F. at the bottom. The volatility modifying agent used is acetonitrile containing a small amount (usually up to about 5 wt. percent) of water. The relative flow rates of the several streams at column 7 are 5,100 #/hr. (stream- 6), 21,000 #/hr. (stream 8), 1,600 #/hr. (stream 9), 3,700 #/hr. (stream 12), 3,300 #/hr. (stream 14) and 1,800 #/hr. (stream 15). The overhead distillate product removed through line 14 and the concentrate or bottoms stream (stream 15) have thecomposition (mol percent) shown in the following table.

TABLE III Stream 14 3 Methyl Butane-l- 2 Methyl Butane-1 Pentene1 Pentadiene4-4 Trans Pentene-2 Cis Pentene-2 2 Methyl Butene-Z- Ois Piperylene Cyclopentadiene 1 Trace.

Stream 15 is charged to the extractive distillation column 16 containing 70 plates and operated at 10 p.s.i.g.

TABLE IV Stream 22 Stream 28 Dimethyl Acetylene 0. 1 valylene. 1. 5 Isoprene 97. 6 33. 7 Trans Piperylene 2. 1 27. 7 Cis Pipe-rylene 0.1 5. 5 Cyclopentadiene 0. 2 31. 5

The isoprene product stream 22 may be subjected to a final straight distillation step to raise the isoprene purity to 99.5 wt. percent.

What is claimed is:

1. An improved method for the separation of isoprene of high purity from a C hydrocarbon fraction containing C monoolefins, C diolefins including cyclopentadiene, dimethyl acetylene and C acetylenes which comprises introducing a mixture of said components directly to means for effecting a straight distillation, subjecting said mixture to a straight distillation to separate a distillate fraction containing isopropyl acetylene, dimethyl acetylene, pentadiene 1 4, valylene and a minor proportion of the isoprene from the feed and a second fraction containing C monoolefins, the major proportion of the isoprene from the feed and higher boiling compounds, subjecting said second fraction to extractive distillation in the presence of a volatility modifying solvent to separate a distillate fraction containing the C monoolefins and a second fraction containing C diolefins and higher boiling C acetylenes and subjecting the latter to a second extractive distillation in the presence of a volatility modifying agent selected from the group consisting of acetone and acetonitrile to separate high purity isoprene as the distillate fraction and a second fraction containing valylene, cyclopentadiene, propylacetylene, cisand trans-piperylene (pentadiene 1-3) and allylacetylene.

2. The process as defined in claim 1 in which the hydro carbon feed fraction is a C fraction from steam cracking.

3. The process as defined in claim 2 in which the volatility modifying agent is acetonitrile containing a small amount of water.

4. The process as defined in claim 3 in which the volatility modifying agent to hydrocarbon ratio is about 1:1

in the first extractive distillation step and about 2:1 in the second extractive distillation step.

5. An improved method for the separation of isoprene of high purity from a C hydrocarbon fraction containing C monoolefins, C diolefins including cyclopentadiene, dimethyl acetylene and C acetylenes which comprises introducing a mixture of said components directly to means for effecting a straight distillation, subjecting said mixture to a straight distillation to separate a distillate fraction containing isopropyl acetylene, dimethyl acetylene, pentadiene-1,4, valylene and a minor proportion of the isoprene from the feed and a second fraction containing C monoolefins, the major proportion of the isoprene from the feed and higher boiling compounds including cyclopentadiene, subjecting said second fraction to extractive distillation in the presence of a volatility modifying solvent selected from the group consisting of acetone and acrylonitrile to separate a distillate fraction containing the C monoolefins and a second fraction containing C diolefins including cyclopentadiene and higher boiling C acetylenes and subjecting the latter to a second extractive distillation in the presence of a volatility modifying agent to separate isoprene of at least 97 wt. percent purity as the distillate fraction and a second fraction containing valylene, cyclopentadiene, propyl acetylene, cisand transpiperylene and allylacetylene, and subjecting the last named distillate fraction to a final straight distillation to obtain an isoprene distillate fraction of about 99.5 wt. percent purity.

6. The process as defined in claim 5 in which the hydrocarbon feed fraction is a C fraction from steam cracking.

7. The process as defined in claim 6 in which the volatility modifying agent is acetonitrile containing a small amount of Water.

8. The process as defined in claim 7 in which the vola tility modifying agent to hydrocarbon ratio is about 1:1 in the first extractive distillation step and about 2:1 in the second extractive distillation step.

9. The process as defined in claim 5 in which said initial straight distillation is effected in a column having about plates and operated at about 36 p.s.i.g. with a temperature of about 138 F. at the top and about 172 F. at the bottom.

10. The process as defined in claim 5 in which said initial straight distillation is efiected in a column having about 70 plates and operated at about 36 p.s.i.g. with a temperature of about 138 F. at the top and about 172 F. at the bottom and wherein the extractive distillation of the second fraction from the initial straight distillation is effected in a column containing about plates operated at about 30 p.s.i.g. with a temperature of about 145 F. at the top and about 255 F. at the bottom.

11. The process as defined in claim 5 in which said initial straight distillation is effected in a column having about 70 plates and operated at about 36 p.s.i.g. with a temperature of about 138 F. at the top and about 172 F. at the bottom, the extractive distillation of the second fraction from the initial straight distillation is effected in a column containing about 100 plates operated at about 30 p.s.i.g. with a temperature of about 145 F. at the top and about 255 F. at the bottom and wherein said second fraction from the extractive distillation is subjected to a second extractive distillation in a column containing about 70 plates operated at about 10 p.s.i.g. with a temperature of about F. at the top and about 215 F. at the bottom.

12. The process as defined in claim 11 in which acetonitrile containing a small amount of water is used as the volatility modifying agent and at a ratio of volatility modifying agent to hydrocarbon of about 1:1 in the first extractive distillation step and about 2:1 in the second extractive distillation step.

References Cited by the Examiner UNITED STATES PATENTS 2,426,705 9/1947 Patterson et a1. 20239.5 2,459,403 1/ 1949 Ahrens 20239.5 2,534,013 12/ 1950 Garrett 20239.5 2,971,036 2/1961 James 202--39.5 3,012,947 12/ 1961 Kelley et a1. 20239.5 3,059,037 10/ 1962 Ca'hn 202-305 X NORMAN YUDKOFF, Primary Examiner. 

1. AN IMPROVED METHOD FOR THE SEPARATION OF ISOPRENE OF HIGH PURITY FROM A C5 HYDROCARBON FRACTION CONTAINING C5 MONOOLEFINS, C5 DIOLEFINS INCLUDING CYCLEOPENTADIENE, DIMETHYL ACETYLENE AND C5 ACETYLENES WHICH COMPRISES INTRODUCING A MIXTURE OF SAID COMPONENTS DIRECTLY TO MEANS FOR EFFECTING A STRAIGHT DISTILLATION, SUBJECTING SAID MIXTURE TO A STRAIGHT DISTILLATION TO SEPARATE A DISTILLATE FRACTION CONTINUING ISOPROPYL ACETYLENE, DIMETHYL ACETYLENE, PENTADIENE 1-4, VALYLENE AND A MINOR PROPORTION OF THE ISOPRENE FROM THE FEED AND A SECOND FRACTION CONTAINING C5 MONOOLEFINS, THE MAJOR PROPORTION OF THE ISOPRENE FROM THE FEED AND HIGHER BOILING COMPOUNDS, SUBJECTING SAID SECOND FRACTION TO EXTRACTIVE DISTILLATION IN THE PRESENCE OF A VOLATILITY MODIFYING SOLVENT TO SEPARATE A DISTILLATE FRACTION CONTAINING THE C5 MONOOLEFINS AND A SECOND FRACTION CONTAINING C5 DIOLEFINS AND HIGHER BOILING C5 ACETYLENES AND SUBJECTING THE LATTER TO A SECOND EXTRACTIVE DISTILLATION IN THE PRESENCE OF A VOLATILITY MODIFYING AGENT SELECTED FROM THE GROUP CONSISTING OF ACETONE AND ACETONITRILE TO SEPARTE HIGH PURITY ISOPRENE AS THE DISTILLATE FRACTION AND A SECOND FRACTION CONTAINING VALYLENE, CYCLOPENTADIENE, PROPYLACETYLENE, CIS- AND TRANS-PIPERYLENE (PENTADIENE 1-3) AND ALLYLACETYLENE. 